domingo, 10 de marzo de 2013

Supernova Cassiopeia A --"A Relativistic Pinball Machine"

This extraordinarily deep Chandra image shows Cassiopeia A (Cas A,
for short), the youngest supernova remnant in the Milky Way. New
analysis shows that this supernova remnant acts like a relativistic
pinball machine by accelerating electrons to enormous energies.

The blue, wispy arcs in the image show where the acceleration is taking
place in an expanding shock wave generated by the explosion. The red and
green regions show material from the destroyed star that has been
heated to millions of degrees by the explosion.
Astronomers have used this data to make a map, for the first time, of
the acceleration of electrons in a supernova remnant. Their analysis
shows that the electrons are being accelerated to almost the maximum
theoretical limit in some parts of Cas A.

Protons and ions, which make up the bulk of cosmic rays,
are expected to be accelerated in a similar way to the electrons.
Therefore, this discovery provides strong evidence that supernova
remnants are key sites for energizing cosmic rays.

Charged particles are believed to scatter or bounce off tangled
magnetic fields in the shock wave, which act like bumpers in a pinball
machine. When the particles cross the shock front they are accelerated,
as if they received a kick from a flipper in a pinball machine.

Typically it should take a few hundred scatterings off the shock's
magnetic field before the particles cross the shock front. It then takes
about 200 crossings of the shock front to accelerate the particles seen
in the Chandra data. Image above shows CasA in the infrared via
NASA/CXC/SAO.

Scientists estimate it would take about 200 years -- over half the
age of the remnant -- to accelerate electrons to cosmic ray energies in
the slowest parts of the shocks, but only about 50 years to accelerate
the highest energy electrons in the regions of maximum acceleration.

A new Chandra X-ray study
of the remains of Cas A indicates that the supernova that disrupted the
massive star may have turned it inside out in the process. Using very
long observations of Cassiopeia A a team of scientists has mapped the
distribution of elements in the supernova remnant in unprecedented
detail. This information shows where the different layers of the
pre-supernova star are located three hundred years after the explosion,
and provides insight into the nature of the supernova.

The predominant concentrations of different elements of Cas A in the
image below of the original star are represented by different colors:
iron in the core (blue), overlaid by sulfur and silicon (green), then
magnesium, neon and oxygen (red). The image uses the same color scheme
to show the distribution of iron, sulfur and magnesium in the supernova
remnant. The data show that the distributions of sulfur and silicon are
similar, as are the distributions of magnesium and neon. Oxygen, which
according to theoretical models is the most abundant element in the
remnant, is difficult to detect because the X-ray emission
characteristic of oxygen ions is strongly absorbed by gas in along the
line of sight to Cas A, and because almost all the oxygen ions have had
all their electrons stripped away.

A comparison of the illustration and the Chandra element map shows
clearly that most of the iron, which according to theoretical models of
the pre-supernova was originally on the inside of the star, is now
located near the outer edges of the remnant. Surprisingly, there is no
evidence from X-ray (Chandra) or infrared (Spitzer Space Telescope)
observations for iron near the center of the remnant, where it was
formed. Also, much of the silicon and sulfur, as well as the magnesium,
is now found toward the outer edges of the still-expanding debris. The
distribution of the elements indicates that a strong instability in the
explosion process somehow turned the star inside out.

This latest work, which builds on earlier Chandra observations,
represents the most detailed study ever made of X-ray emitting debris in
Cas A, or any other supernova remnant resulting from the explosion of a
massive star. It is based on a million seconds of Chandra observing
time. Tallying up what they see in the Chandra data, astronomers
estimate that the total amount of X-ray emitting debris has a mass just
over three times that of the Sun. This debris was found to contain about
0.13 times the mass of the Sun in iron, 0.03 in sulfur and only 0.01 in magnesium.

The researchers found clumps of almost pure iron, indicating that
this material must have been produced by nuclear reactions near the
center of the pre-supernova star, where the neutron star was formed.
That such pure iron should exist was anticipated because another
signature of this type of nuclear reaction
is the formation of the radioactive nucleus titanium-44, or Ti-44.
Emission from Ti-44, which is unstable with a half-life of 63 years, has
been detected in Cas A with several high-energy observatories including
the Compton Gamma Ray Observatory, BeppoSAX, and the International Gamma-Ray Astrophysics Laboratory (INTEGRAL)